Just follow the simple steps Below:
1) Right click on the Google Talk shortcut.
2) click on Properties.
3) Go to Shortcut tab on Google Talk Properties window.
4) On the Target textbox, add in the /nomutex to the end of the line so
that it looks like below (or you can simply copy and paste the below
syntax and replace the original).
“c:\program files\google\google talk\googletalk.exe” /nomutex
5) Click on OK.
I didn't have to do anything after this and clicking on the shortcut
multiple times just gave me different Google talk window.

Alternative

To create a new shortcut for Google Talk:

1) Right-click on the desktop or anywhere you want to place the GTalk
shortcut.
2) Select New on the right click context menu.
3) Then select Shortcut.
4) Copy and paste the following line to the text box when prompted to type
the location of the item:
“c:\program files\google\google talk\googletalk.exe” /nomutex

5) Click on Next.
6) Give the shortcut a proper name such as Google Talk or Google
Talk Multiple or Google Talk Polygamy.
7) Click OK until you are done.